1. Field of the Invention
The present invention relates to transformers for electrical power distribution systems, power supplies, electromagnetic machinery and the like; and, more particularly, to a current transformer for precision measurement of electrical current, in which the core material responds linearly to the level of magnetic excitation.
2. Description of the Prior Art
Direct measurement of electrical current flowing in a conductive media such as copper wire is not straightforward, especially when the current level and the voltage at the media are high. Indirect measurement methods include conventional electrical meters based on monitoring eddy current generated by an electrical current flow, use of current dividers in which a low current flowing section is comprised of a precision resistor, and magnetic flux meters detecting changes in the magnetic fields generated by an electrical current flow. All of these techniques have drawbacks. For example, eddy-current based conventional electrical meters are not accurate, especially when the current to be measured contains higher harmonics of the fundamental current frequency. The current dividers are hazardous when the current line voltage is high. Magnetic flux meters are widely used, in which the flux generated by a current is detected by a Hall effect sensor or a sensing coil. In both cases, a flux concentrator with a high magnetic permeability is generally utilized to improve sensitivity. To achieve a high degree of accuracy, the magnetic permeability has to be such that the magnetic flux generated in the flux concentrator is directly proportional to the magnetic field caused by the current to be measured. Such a magnetic concentrator is usually a soft magnetic material having a highly linear B-H characteristic where B is the magnetic flux density and H is the magnetic field generated by an electrical current flowing orthogonally with respect to the direction of the magnetic flux.
A linear B-H characteristic is generally obtained in a soft magnetic material in which the material's magnetically easy axis lies perpendicular to the direction of the magnetic excitation. In such a material, the external magnetic field H tends to tilt the average direction of the magnetic flux B such that the measured quantity B is proportional to H. Since the field H is proportional to the electrical current to be measured, the flux B is directly proportional to the current. Most of the magnetic materials, however, have nonlinear B-H characteristics and ideal linear B-H characteristics are difficult to achieve. Any deviation from an ideal B-H linearity introduces inaccuracies in the measurement of electrical current using magnetic flux meters.
A classical example of magnetic materials showing linear B-H characteristics is a cold rolled 50% Fe—Ni alloy called Isoperm. Among amorphous magnetic alloys, heat-treated Co-rich alloys have been known to provide linear B-H characteristics and are currently used as the magnetic core materials in current transformers. The Co-rich amorphous alloys in general have saturation inductions lower than about 10 kG or 1 tesla, which limits the maximum current levels to be measured. Besides, these alloys are expensive owing to the large amount of Co used to form the alloys. Clearly needed are inexpensive alloys having saturation inductions higher than 10 kG (1 tesla), which exhibit linear B-H characteristics.
Amorphous metal alloys have been disclosed in U.S. Pat. No. 3,856,513, issued 24 Dec. 1974 to Chen and Polk. These alloys include compositions having the formula MaYbZc, where M is a metal selected from the group consisting of iron, nickel, cobalt, vanadium and chromium, Y is an element selected from the group consisting of phosphorous, boron and carbon and Z is an element selected from the group consisting of aluminum, silicon, tin, germanium, indium, antimony and beryllium, “a” ranges from about 60 to 90 atom percent, “b” ranges from about 10 to 30 atom percent and “c” ranges from about 0.1 to 15 atom percent. Also disclosed are amorphous metal wires having the formula TiXj, where T is at least one transition metal and X is an element selected from the group consisting of phosphorus, boron, carbon, aluminum, silicon, tin, germanium, indium, beryllium and antimony, “i” ranges from about 70 to 87 atom percent and “j” ranges from 13 to 30 atom percent. Such materials are conveniently prepared by rapid quenching from the melt using processing techniques that are well known in the art.
These disclosures mention unusual or unique magnetic properties for many amorphous metal alloys, which are generally discussed and defined therein. However, amorphous metal alloys possessing a combination of linear BH characteristics and the saturation inductions exceeding about 10 kG (1 tesla) are required for specific applications such as current/voltage transformers.